The invention relates to an antidazzle device for charge transfer image sensors, such as those used in television cameras. In such an image sensor, a row or matrix of photosensitive elements receives light bearing an image. This light releases charges in each photosensitive element proportionally to the amount of light received by this element. These charges are accumulated in the photosensitive elements during a so called accumulation phase, then are transferred to a charge transfer register, called reading register, during a so called reading phase. The charges accumulated by a row of photosensitive elements are transferred in parallel into the reading register, then are restored sequentially; the role of the reading register being to shift the charge packet corresponding to each photosensitive element towards a conversion capacity whose role is to deliver an electric voltage proportional to the amount of charges in each packet. This voltage is then amplified by a preamplifier whose output forms the output of the image sensor.
It frequently happens that an image includes over illuminated zones, because they represent a light source or an object reflecting a light source, situated in the picture taking field of the image sensor. The amount of charges released in the overilluminated photosensitive elements may saturate these elements in the reading register. The excess charges may overflow into adjacent photosensitive elements and the adjacent cells of the reading register. This phenomenon results in a halo or a light trail in the vicinity of the overilluminated points, called dazzle, in the image restored from the signal delivered by the image sensor.
It is not possible to avoid overillumination of certain photosensitive elements, without exaggeratedly reducing the illumination of the whole of the image sensor. To avoid the dazzle phenomenon of an image sensor, numerous devices, called antidazzle devices, have been designed. A first known type of device includes an antidazzle drain close to each photosensitive element and separated therefrom by a potential barrier whose algebraic value is greater than that of the potential barrier separating the photosensitive element from the reading register during the accumulation phase. Thus the drain removes the excess charges which could overflow from this photosensitive element during the accumulation phase, in the presence of overillumination.
A second known type of antidazzle device includes an antidazzle drain situated in depth under each photosensitive element. This drain is manufactured in accordance with the technique called buried layer or caisson technique. The operating principle is identical to that of the device of the first type.
Antidazzle drains are perfectly efficient during the charge accumulation time but are totally inoperative during the time for transferring the charges into the reading register, for the potential barrier defining each cell of the reading register has a level less than that of the potential barrier separating the photosensitive element from its antidazzle drain. The reading phase is of a much shorter duration than the accumulation phase, nevertheless high overillumination may cause cell saturation of the reading register. The efficiency of these two types of antidazzle device is therefore limited to moderate over illumination. It is not possible to simply reduce the generation of the reading phase to combat this dazzle phenomenon, for a reduction in this duration of the reading phase may cause an increase of the remanence phenomena, resulting in a degradation of the vertical modulation transfer function of the image sensor.
French patent application No. 2 538 200 describes an image sensor having a second type of antidazzle device, in which the duration of the reading phase is reduced without causing degradation of the vertical modulation transfer function by using a so called entraining charge, which is introduced into the reading register through a so called injection stage. The entraining charge is transferred to the cells of the reading register during the accumulation time of the charges released by the light and removal of the previously accumulated charges, then is transferred into the photosensitive element just before reading of the charges accumulated in this element. The charges released by the light and the entraining charge are then transferred together, from the photosensitive element to the cell of the reading register, during the reading phase whose duration is reduced.
The entraining charge has a constant value which, theoretically, does not disturb the value of the video signal delivered by the image sensor. However, the entraining charge adds a so called injection noise and a so called transfer noise to this video signal. The use of a volume transfer, rather than a surface transfer reading register does not increase the noise of the image sensor too much, however this noise is greater than that of a conventional image sensor. Furthermore, this device extends the limit of acceptable over-illumination without dazzle, but not sufficiently.